The pre-Fermi natural reactor

The origin of the concept of a large-scale nuclear chain reaction occurring in nature can be traced back to the ideas expressed byAston in 1922 and byJoliot in 1935. Geochemical investigations on hot springs, which have been carried out at the University of Tokyo since the 1930s, played a key role in the early development of the theory of natural reactor. Results obtained from the studies, which have been carried out in various countries since the 1972 discovery of the Oklo phenomenon, reveal the fact that the natural reactors at Oklo may have indeed operated in a manner quite similar to the geysers or intermittent hot springs. A careful examination of the isotopic compositions of the so-called anomalous xenon from the Oklo reactor suggests that the natural reactors were operating at temperatures between the boiling point of iodine (183°C) and the melting point of tellurium (452°C), periodically being turned on and off.